occurances of a letter in a string

#include <iostream>
#include<cctype>
using namespace std;
void occurence(string s)
{
int letters[26]={0};
for(char ch:s)
{
ch=tolower(ch);
if(isalpha(ch))
{
letters[ch-'a']++;

}

}
for(int i=0;i<26;i++)
{
if(letters[i]>0)
{
char letter='a'+i;
cout<<letter<<" "<<letters[i]<<endl;
}
}
}

int main() {
string s;
getline(cin,s);
occurence(s);
return 0;
}